1. Field of the Invention
The invention relates to a signal processing apparatus suitable for use in a television set.
2. Description of the Related Art
A block diagram of a conventional signal processing apparatus is shown in FIG. 1. First in FIG. 1, a text signal or a personal computer signal (hereinafter, simply referred to a text signal) of characters or the like comprising analog luminance signal Y and color difference signals U and V is inputted from input terminals 51Y, 51U, and 51V. The inputted luminance signal Y and color difference signals U and V are supplied to A/D converters 52Y, 52U, and 52V. In the A/D converters 52Y, 52U, and 52V, the supplied luminance signal Y and color difference signals U and V are converted into digital signals at predetermined sampling frequencies and the text signal of the digital component signals is supplied to a text signal processing circuit 53. A ratio of the sampling frequencies in the A/D converters 52Y, 52U, and 52V is set to (Y:U:V)=(4:4:4).
A field memory 54 is connected to the text signal processing circuit 53. In the text signal processing circuit 53, predetermined signal processes are performed to the luminance signal Y and color difference signals U and V. The signal processed luminance signal Y and color difference signals U and V are supplied to a mixing circuit 55.
A video signal comprising the analog luminance signal Y and color difference signals U and V is inputted from input terminals 56Y, 56U, and 56V. The inputted luminance signal Y and color difference signals U and V are supplied to A/D converters 57Y, 57U, and 57V. In the A/D converters 57Y, 57U, and 57V, the supplied luminance signal Y and color difference signals U and V are converted into digital signals at predetermined sampling frequencies. A video signal of digital component signals is supplied to a video signal processing circuit 58. A ratio of the sampling frequencies in the A/D converters 57Y, 57U, and 57V is set to (4:1:1) or (4:2:2).
A field memory 59 is connected to the video signal processing circuit 58. In the video signal processing circuit 58, predetermined signal processes are performed to the luminance signal Y and color difference signals U and V. The signal processed luminance signal Y and color difference signals U and V are supplied to the mixing circuit 55.
In the mixing circuit 55, the text signal of (4:4:4) and the video signal of (4:1:1) or (4:2:2) are, for instance, switched and mixed. An output of the mixing circuit 55 is supplied to D/A converters 60Y, 60U, and 60V.
In the D/A converters 60Y, 60U, and 60V, the luminance signal Y and color difference signals U and V are converted into analog signals. The analog luminance signal Y and color difference signals U and V are supplied to LPFs (low pass filters) 61Y, 61U, and 61V. In the LPFs 61Y, 61U, and 61V, filter processes are performed to the luminance signal Y and color difference signals U and V in order to remove unnecessary signals. The filter processed luminance signal Y and color difference signals U and V are outputted from output terminals 62Y, 62U, and 62V.
To clearly display characters, in the text signal, it is assumed that the color difference signals U and V and luminance signal Y have a similar band. Therefore, the ratio of the sampling frequencies is set to (4:4:4). On the other hand, in the video signal, a ratio of the sampling frequencies is set to (4:1:1) or (4:2:2) because the band of the color difference signals is narrower than that of the luminance signal Y. Thus, the capacity of the memory that is necessary for the signal processes can be saved. For the purpose of commonly using the D/A converter, two signals of the text signal of (4:4:4) and the video signal of (4:1:1) or (4:2:2) are mixed by digital signals prior to D/A converting. The band of the LPF after the D/A converter is matched with the band of the text signal together with the luminance signal Y and color difference signals U and V.
However, in the conventional system, since the band of the LPF at the post stage of the D/A converter is set to the wide band according to the band on the text signal side, the high frequency components (the harmonics components) of the color difference signals on the video signal side cannot be sufficiently attenuated. For example, as shown in FIG. 2, there is a problem such that oblique color edges which draw a diagonal line become a stairway shape.
On the other hand, if the band of the LPF is matched with the narrow band of the color difference signal of the video signal, since the high component of the text signal is eliminated, the color difference signals of the text signal become dull and there is a problem such that a color blur occurs and it is very hard to read the text.
It is, therefore, the invention to provide a signal processing apparatus to which a band limiting filter which can sufficiently attenuate the high frequency components of the color difference signals of the video signal is applied.
The invention is provided a signal processing apparatus for mixing a first digital component signal and a second digital component signal in which a sampling frequency of a color component is lower than that of the first digital component signal, characterized by comprising: a digital filter for performing a band limitation to the color component of the second digital component signal; mixing means for mixing the first digital component signal and the second digital component signal; D/A converting means for D/A converting an output of the mixing means; and a filter for performing a band limitation to an analog signal converted by the D/A converting means.
According to the invention, the high frequency components are removed from the color difference signals of the video signal (second digital component signal) of the ratio of the sampling frequencies of (4:2:2) or (4:1:1) by using a digital band limiting filter. After that, those signals are mixed and the mixed signal is D/A converted. Therefore, even if the band of the LPF at the post stage of the D/A converter is set to the wide band according to that of the text signal or the personal computer signal (first digital component signal) of the ratio of the sampling frequencies of (4:4:4), the oblique color edges are smoothly displayed.
The above, and other, objects, features and advantage of the present invention will become readily apparent from the following detailed description thereof which is to be read in connection with the accompanying drawings.